why is it " fresa rojo" instead of "rojo fresa"
why is the name of a color used after a subject in a spanish sentence
Basically because Spanish is structured differently than English. For most (but not all) cases the descriptive adjective will follow the noun rather than precede it. Here are a couple of links that you might find useful:
Try the free video lessons and have a look at the reference section while you are here. There is a ton of useful information here for the taking.
Do you mean after a noun?
I'm not sure that it does always follow the noun, but it does in the majority of the cases. The reason is that colors are considered descriptive adjectives.